Research On Critical Excitation Direction And Support Displacement Control Of The Isolated Curved Girder Bridge Under Near Fault Ground Motion

The isolated curved girder bridges are widely used because of their good adaptability and passing ability in the face of complex terrain.However,curved lines will cause the upper structure to be affected by curvature and torsion during earthquakes,resulting in complex forces on the lower structure.Moreover,isolated bearings as well as the participation of complex excitation of near-fault ground motion make it possible to calculate the most unfavorable input direction of the isolated curved girder bridge only by time-history analysis method with full-angle input,but this method takes a long time and is difficult to be applied in engineering design.At the same time,the impulse characteristics of near-fault ground motion have a great influence on the displacement of the isolated curved beam bridge support,which makes it easy to exceed the limit value in earthquake.Therefore,in this paper,the most unfavorable input direction of the isolated curved girder bridge and the control of displacement of the isolation bearing under near-fault ground motion are studied as follows:(1)Quantitative identification method of energy-based pulsed ground motion is used to screen out near-fault ground motion and carry out spectrum comparison analysis with far-field ground motion.The results show that the selected near-fault pulsed ground motion contains abundant long-period components and the pulsed part accounts for most of the energy of ground motion,which is easy to damage the isolated bridge.(2)The isolated curved girder bridge numerical model with friction pendulum bearings is established by finite element software,and the screened seismic full-angle input model is used for non-linear time-history analysis to determine the most unfavorable input angle of the bridge under near-fault seismic motion,and to analyze the influence of near-fault seismic pulse characteristics on the most unfavorable response of the isolated curved girder bridge.The results show that the friction pendulum bearings move far beyond the limit under nearfault pulsed ground motion,and the pier internal force can not meet the seismic design requirements.(3)Three simplified calculation methods for the most unfavorable input direction of curved girder bridges Based on SRSS mode combination,CQC mode combination response spectrum method and time history analysis method are selected.By comparing the results of full-angle input time history analysis method,the applicability of the three simplified methods in the isolated curved girder bridges is studied.The results show that all the three methods are suitable for determining the most unfavorable input direction of the isolated curved girder bridge.(4)In order to solve the problem of excessive displacement of the isolated curved girder bridge support under near-fault pulsed ground motion,a hybrid isolation scheme of viscous damper and friction pendulum bearings is adopted.The arrangement of viscous damper and the influence of parameter setting on seismic response of the isolated curved girder bridge are studied,and the damping effect of the hybrid isolation scheme is comprehensively evaluated.The results show that the hybrid isolation scheme can control the support displacement,optimize the internal force response of curved girder bridge and make up for the shortage of single isolated measures.